NOTE: These are the main F.E.A.R. cheats that are usually applied by pressing down the right key combination. Please find the instructions on how to use these cheats below.

All levels unlocked
Enter F3ARDAY1 as your PROFILE NAME. NOTE: Achievements will be DISABLED if you use this profile name.

F3ARDAY1: Unlocks all levels/Achievements ARE disabled.

Hints

NOTE: As their name suggests hints are not exactly F.E.A.R. cheats, and are usually just simple (but very useful) tips on how you can unlock secret level, features, find hidden areas and such. Hopefully these will make playing F.E.A.R. even more entertaining.

Achievements
Do the following tasks to unlock the following achievements and thier alloted gamerscore:

Ammo Hog (35): Win the Campaign using less than 500 rounds of ammo.
Armacham (5): Complete Interval 7 in the Campaign.
Assaulter (5): Kill 50 enemies with the G2A2 in any ranked Multiplayer games.
Belly Gunner (5): Kill 50 enemies with the turret in any ranked Multiplayer games.
Black Belt (5): Kill 50 enemies with Round House Kicks, Flying Kicks, or Slide Tackles in any ranked MP games.
Brawler (5): Kill 50 enemies with Punches or Rifle Butts in any ranked Multiplayer games.
Bullet Sponge (5): Use 50 armor vests in the Campaign or Instant Action.
Dead Eye (35): Kill 150 enemies with Headshots in any ranked Multiplayer games.
Demolitions Expert (35): Kill 200 enemies with the N6A3, AT-S or M77 in any ranked Multiplayer games.
Disintegrator (5): Kill 50 enemies with the Type-7 in any ranked Multiplayer games.
Dual Machinist (5): Kill 50 enemies with the SM15 in any ranked Multiplayer games.
Environmentalist (5): Kill 10 enemies with barrels, fire extinguishers or fuse boxes in the Campaign or Instant Action.
Explorer (35): Find every Health Booster and Reflex Booster in the single player campaign.
Explosives Expert (5): Kill 3 or more enemies with a single N6A3, AT-S or M77 in the Campaign or Instant Action.
Exterminator (35): Kill every enemy in the Campaign.
Feared (5): Complete every Instant Action map on any difficulty except Low.
Fearless (35): Win the Campaign on any difficulty except Low.
Fearless Flagman (75): Successfully capture 50 flags in ranked CTF or SlowMo CTF games.
Fearsome (100): Complete the Campaign on Extreme difficulty.
Grenadier (5): Kill 50 enemies with the N6A3 in any ranked Multiplayer games.
Initiation (5): Complete Interval 2 in the Campaign.
Intel (5): Gather every phone message and lap top in the Campaign.
Medic! (5): Use 50 Medkits in the Campaign or Instant Action.
MP 100 (5): Participate in 100 rank Multiplayer games of any type.
MP 1000 (75): Participate in 1000 ranked Multiplayer games of any type.
Multiplayer Victory (75): Win any ranked Deathmatch, SlowMo Deathmatch, or Elimination Multiplayer game.
No Fear (35): Win every Instant Action map on Extreme difficulty.
No Juice (65): Complete the Campaign on any difficulty without using any boosters.
Perforator (5): Kill 50 enemies with the 10mm HV Penetrator in any ranked Multiplayer games.
Pistoleer (5): Kill 50 enemies with the AT-14 in any ranked Multiplayer games.
PumpShotty Ownage (5): Kill 50 enemies with the VK-12 in any ranked Multiplayer games.
Real Time (65): Complete the Campaign on any difficulty without using SlowMo.
Repeating Offender (5): Kill 50 enemies with the MP-50 in any ranked Multiplayer games.
Rocketman (5): Kill 50 enemies with the MOD-3 in any ranked Multiplayer games.
Sapper (5): Kill 50 enemies with the M77 in any ranked Multiplayer games.
Secret Facility (5): Complete Interval 10 in the Campaign.
Slow Jo (5): Kill 5 enemies with one SlowMo use in the Campaign or Instant Action.
Slums (5): Complete Interval 8 in the Campaign.
Sniper (5): Kill 50 enemies with the ASP in any ranked Multiplayer games.
Sub-Jugator (5): Kill 50 enemies with the RPL in any ranked Multiplayer games.
Survivalist (150): Complete the Campaign on any difficulty without dying.
Team Victory (5): Win 5 each of ranked CTF, SlowMo CTF, TDM, SlowMo TDM, and Team Elimination games in Multiplayer.
Trap Layer (5): Kill 50 enemies with the AT-S in any ranked Multiplayer games.
Water Treatment Facility (5): Complete Interval 3 in the Campaign.

Saving Medkits
Make a mark of some sort to lead you back on some Medkits that you could not carry before. If you so happen to run out of your own because of a big fight you can find them again. This is useful in Interval 11 with the armies of ghosts coming after you. Just make sure that you are able to backtrack to get the Medkits you left behind.

Use your Medkits at the correct time. Do not use Medkits immediately when you have about 70 to 80 health remaining. Try using a Medkit when your health drops below 30. One Medkit can heal up to about 30 to 45 points. If your health happens to drop below 40 and you are in a 1v1 or 2v1 fight, do not bother using a Medkit unless you really need to. Just run to cover while using your slow-mo abilities. Your health will regenerate slowly back to 40 if it drops below that, however you must be out of the firefight to regenerate your health.

Defeating Assassin Super Soldiers
Because the assassins are difficult to see due to their camouflage and high speed running, try using slow-motion with L2 PlayStation3 or LB Xbox 360 . You will notice them immediately once they are slowed. After that, let all of your rounds into them.

If you do not have slow-motion (for example, in the bonus mission when playing as the demolition man), you must have a quick finger and a good eye to notice them running around. There are two ways to do this. The first is to toss a grenade to reveal the assassin for a certain amount of time. Hopefully the splash radius will either hit him or kill him. Note: The grenade has a two to four second delay before exploding, and correct timing is required. The second way is to wait until the assassin approaches directly in front of you. When he reveals himself, start firing at him.